The present invention relates to a thermal type air flow meter which uses a heating resistor to measure the fluid flow rate such as an intake air flow in an internal combustion engine used, for example, for an automobile.
As a flow amount measuring device by which the intake air flow of the internal combustion engine such as automobiles is measured, a device which measures flow amount according to the heat radiation amount from a heating resistor which corresponds to flow amount, obtained by heating the heating resistor is known in general. Further, a thermal type air flow meter which measures flow amount by heating a heating resistor, and detecting the change in temperature of a temperature sensitive resistor arranged in the neighborhood of the heating resistor is also known.
Electronic circuit parts (electronic components) which drive the flow amount detection element including a heating resistor and the flow amount measuring device are mounted on the supporting substrate (support member such as circuit boards) made of electrical insulating material with large thermal conductivity such as ceramic etc.
The miniaturization of the flow amount measuring device advances with the object such as decreasing the cost and the number of parts of the flow amount measuring device. In recent years, the flow amount detection element has been mounted on the position near the electronic circuit parts. Therefore, it is easier to conduct heat to the flow amount detection element by the self-heating of the electronic circuit parts, thereby increasing the heat as a factor in measurement error.
In order to solve this problem, heat has in the past been radiated outside by putting the metallic plate on the circuit board where the electronic circuit parts are mounted and exposing a cooling wheel of the metallic plate in a fluid channel (as seen, for instance, in Japanese Patent Application Laid-Open No. 5-231899). Further, it is also known to mount the flow amount detection element on the circuit board where the electronic circuit parts are mounted. The window or the notch part is formed between the two, and the part where the electronic circuit of the circuit board is mounted and the part where the flow amount detection element is mounted thermal are thermally shielded (as seen, for instance, in Japanese Patent Application Laid-Open No. 9-145440).
In the prior art, cooling has been carried out just from one direction where air flows. It cannot suppress the heat influence on the flow amount detection element because of the low cooling efficiency. The accuracy of measurement is especially deteriorated in the miniaturized thermal type air flow meter.
Further, the influence of the heat of the main passage besides the heat influence by the self-heating of the electronic circuit parts is received in this known kind of thermal type air flow meter. Therefore, the heat influence cannot be suppressed enough, and measures of the more efficient cooling and the heat separation are needed.
The heat of the main passage is conducted to the flow amount detection element when the temperature of the fluid which flows in the inside is low though the temperature of the main passage is high, and thus the measurement error increases. For instance, although the temperature of the outside air may be low and thus the temperature of the intake air in the intake pipe also low, the intake pipe itself is warm because it receives the heat of the engine in the intake pipe for an automobile engine. Because the heat of the main passage (in this case, an intake pipe) is conducted to the flow amount detection element through the housing case and the support body of the flow amount detection element in such a state, the conducted temperature becomes the influential factor in the measurement error.
Further, the metallic plate might corrode when the metallic plate is exposed in the fluid channel as a cooling wheel in the above-mentioned prior art. Further, in practical use, mechanical strength decreases when the window or the notch part is formed in the circuit board.
An object of the present invention is to provide a thermal type air flow meter which can measure accurately the flow amount without being influenced by the self-heating of the electronic circuit parts and the heat of the main passage by cooling the route of thermal conduction to the flow amount detection element efficiently.
According to one characteristic of the present invention, a support member where a flow amount detection element including a heating resistor and electronic circuit parts are mounted is cooled from both the direction of the main flow of main passage and the direction of an anti-main flow (i.e., the direction opposite to the direction of the main flow).
Concretely, the present invention includes a housing for accommodating the electronic circuit parts mounted on the support member, and an auxiliary passage body which forms a flow path for cooling the support member by the fluid which flows in the direction of the main flow of the main passage, and an auxiliary passage with an opposite-direction passage portion where the fluid flows in the opposite direction to the direction of the main flow of the main passage. The fluid detection element mounted on the support member is provided in the opposite-direction passage portion. The auxiliary passage having a curved portion, which is usually called the curved auxiliary passage, is formed in the auxiliary passage body of the present invention.
Because the support member where the flow amount detection element including a heating resistor and electronic circuit parts are mounted is cooled from both the direction of the main flow of main passage and the direction of an anti-main flow (i.e., the direction opposite to the direction of the main flow) in the present invention, it is now possible to measure accurately the flow amount without being influenced by the self-heating of the electronic circuit parts and the heat of the main passage by cooling the route of thermal conduction to the flow amount detection element efficiently.